CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (103,916)	$ (55,577)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	6,318	1,289
Stock-based compensation expense	7,635	2,623
Amortization of premium on short-term investments	(1,871)	(1,589)
Loss on disposal of property and equipment	13	127
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,759	(5,003)
Accounts payable	(4,452)	4,383
Accrued expenses and other liabilities	2,315	3,042
Deferred revenue	(569)	(1,890)
Deferred rent	410	10,033
Net cash used in operating activities	(91,358)	(42,562)
Cash flows from investing activities:
Purchases of short-term investments	(286,391)	(245,328)
Maturities of short-term investments	256,425	148,480
Purchases of property and equipment	(21,833)	(24,939)
Net cash used in investing activities	(51,799)	(121,787)
Cash flows from financing activities:
Proceeds from issuance of common stock in initial public offering, net of discounts and issuance costs		150,843
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs	134,524
Proceeds from issuance of common stock from option exercises	351	87
Proceeds from issuance of common stock pursuant to employee stock purchase plan	829
Proceeds from issuance of restricted common stock		64
Net cash provided by financing activities	158,213	150,994
Net change in cash, cash equivalents and restricted cash	15,056	(13,355)
Cash, cash equivalents and restricted cash, beginning of period	39,992	53,347
Cash, cash equivalents and restricted cash, end of period	55,048	39,992
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	60	75
Property and equipment additions included in accounts payable	455	9,127
Property and equipment additions included in accrued expenses and other liabilities	249
Unrealized gain on available for sale securities, net	260	(4)
Offering costs included in accrued expenses and other liabilities	100
ATM
Cash flows from financing activities:
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs	$ 22,509
Series A Convertible Preferred Stock | Initial Public Offering
Supplemental disclosures of noncash investing and financing activities:
Conversion of convertible preferred stock into common stock upon initial public offering		42,994
Series B Convertible Preferred Stock | Initial Public Offering
Supplemental disclosures of noncash investing and financing activities:
Conversion of convertible preferred stock into common stock upon initial public offering		$ 94,768